I just did a quick search, and found this:

Concurrent use of stimulants with antidepressants is a high-risk behavior and is medically contraindicated9 due to potential cytochrome P450 2D6 (CYP2D6) enzyme inhibition and a resulting increase in serotonin levels.

www.ncbi.nlm.nih.gov/pmc/articles/PMC4309786/

While it doesn’t specifically name Zoloft (Sertraline), it seems to boil down to how the body uses its own enzymes needed to metabolize drugs. You can end up getting pronounced effects from both drugs because they use/affect the same enzymes.